1. Field of the Invention
The present invention relates to a color laser display apparatus having a fluorescent screen scanned with laser light.
2. Description of the Related Art
High-power laser light sources having output power in the order of watts are needed for increasing luminosity of color laser display apparatuses. Conventionally, high-power Ar+ gas lasers or Kr+ gas lasers have been used as the laser light sources in the color laser display apparatuses. However, the laser light sources using gas lasers have low energy conversion efficiency, which is typically 0.1%, and require a water cooling system. In addition, the size and cost of the laser light sources using the gas lasers are great.
As reported in Laser Focus World (December 1997) p.52, recently, laser-diode-excited SHG solid-state lasers have been used as a short-wavelength visible laser light source. For example, SHG solid-state lasers which emit laser light of a green wavelength of 532 nm have higher energy conversion efficiency than those of the gas lasers, where YAG lasers which emit laser light having a wavelength of 1,064 nm are used in the SHG solid-state lasers.
However, in the above SHG solid-state lasers, the excitation laser light is visible to human eyes. Therefore, color purity of the display is affected by the excitation laser light.
In addition, in the laser-diode-excited SHG solid-state lasers, a kind of noise, called longitudinal mode competition, occurs when the output power is increased. For example, Laser Focus World (May 1998) p.243 reports that red solid-state lasers generate as much as 50% noise while noise generated in blue or green solid-state lasers generate is less than 3%. In order to control the longitudinal mode, an etalon may be used as a wavelength selection element. However, if single-mode oscillation is realized by using an etalon, large energy loss occurs. Therefore, it is impossible to obtain high power output and high luminosity.
As described above, when the above laser-diode-excited SHG solid-state lasers are used as a laser light source in the color laser display apparatuses, the improvement is made in the size and the energy conversion efficiency. However, there are still problems of performance and cost.